In recent years, Innate Immunity has gone from being the immunologists' dirty little secret to being among the most active and exciting areas of immunology. Many recognition molecules of vertebrate innate immune cells have been defined and much is now known about their mechanisms of action. Nonetheless, much remains to be learned before we truly understand how to harness these mechanisms for vaccination and cancer immunotherapy or how to block them to treat autoimmune and inflammatory disease. This proposed Program Project combines 4 investigators with established expertise in the area of innate immunity to pursue related studies developing out of their independent research efforts, but containing numerous connections and great potential for combined effort. In Project #1, Dr. DeFranco will utilize his newly created conditional allele of myd88 to dissect the cellular basis of Toll-like receptor signaling for systemic and mucosal immune responses, with emphasis on airways and fungal infections. In Project #2, Dr. Ma will analyze the role of the ubiquitin-modifying regulator A20 in dendritic cells for restraining TLR responses and preventing inflammatory disease. In Project #3, Dr. Lowell will determine the mechanism by which myeloid cells contribute to lupus-like autoimmunity in the Lyn-deficient mouse model. Finally, in Project #4, Dr. Locksley will determine how chitin, a polysaccharide found in fungi and invertebrates, promotes type 2 immunity and how it interacts with TLR signaling pathways to regulate the type of immune response. Lay Language: The immune system recognizes conserved elements of viruses, bacteria, fungi and multicellular invertebrates to allow it to detect infections and fight them. Immunologists are defining a number of the molecular mechanisms by which this is done, but much remains to be learned, particularly to understand how these reactions are controlled to avoid excessive inflammation and tissue injury, while directing the immune system toward the type of immune response most beneficial for fighting the infectious agent that is present. Better understanding of these issues will lead to improved vaccination procedures and better ability to control excessive inflammatory conditions.